Metal oxide semiconductor gas sensors are well known and have been used for many years. These sensors apply the principle that when a metal oxide semiconductor is exposed to a particular reactive gas, its electrical resistance changes as a function of the concentration of the reactive gas.
Typically, these gas sensors have been used to detect, among other things, oxygen in fuel exhaust systems to determine the fuel to air ratios for improved efficiency in combustion engines, as well as determining dangerous concentrations of poisonous gases in mine shafts, welding operations, and other areas where certain gases such as methane, H.sub.2 S and CO can present potential dangers. There have been a number of gas sensors which have been developed over the years utilizing a variety of materials and designs. These designs generally consist of a semiconductor material, electrical leads, a power source, and a heating device of some sort. These sensor designs to date have been relatively complex. Certain designs require that the semiconductor be comprised of a conducting and nonconducting metal oxide mixture as in U.S. Pat. Nos. 3,865,550; 3,955,268 and 4,013,943. Other designs have required that the metal oxide semiconductor material be supported on a substrate of some sort, U.S. Pat. No. 4,338,281. And still others have configurations which require temperatures in excess of 700.degree. C or have current requirements greater than 150 mA, U.S. Pat. Nos. 3,886,785; 4,004,452 and 3,955,268. Many of these gas sensors have found their greatest utility in portable gas detecting units. However, the prior art sensing units have a number of drawbacks when applied in these units. Primarily, it is the fact that they require a significant amount of power to operate which does not make them efficient in battery powered devices. Their high power consumption is due in general to a number of reasons, one reason being the high temperatures at which these sensors operate requiring greater current flow through the heater. Another is the poor design which results in bulky, high mass sensors or which require more power to heat them to their operating temperatures. In many instances, this is due to the inefficient design which ultimately leads to a larger than necessary mass for the sensor requiring more energy to maintain their operating temperature. Since these sensors must be maintained at a relatively high temperature, the greater the mass, the more current must be used to heat them. This increase in power drain makes these sensors less desirable for use in small portable units.
Therefore, what is needed in the art is a low power consuming, metal oxide semiconductor gas sensor, whose power requirements are significantly lower than the present gas sensors. Such a sensor would make a truly portable gas sensing system practical.